The present invention pertains to methods of multi-stage or two-stage washing-out of ammonia from gas, particularly from coking oven gas.
Multi-stage or two-stage methods for the removal of ammonia from coking oven gas have been known in the art. One of the known methods of the type under discussion includes the steps of washing or scrubbing coking oven gas with water enriched with ammonia at one stage of the method and washing that gas with ammonia-free water at another stage.
It is known from so-called indirect method for collecting ammonia from coking oven gas that ammonia contained in gas in conjunction with gas cooling is washed out from the gas at the first or second stage of the ammonia removal process by means of water enriched with ammonia utilized as a washing medium. This water enriched with ammonia utilized at the first stage of the process can be further again used at the following washing stages, furthermore, ammonia-containing condenser or cooler condensate freed from tar can be also utilized. The residuals of ammonia are then washed out with ammonia-free water at the last stage of the washing process unless gas contains only about 2 g of NH.sub.3 per 100 m.sup.3. A suitable softened fresh water is normally utilized at that last washing stage. By processing water in an ion exchanger device calcium and magnesium ions acting as hardening constituents are substituted by sodium ions to obtain soft fresh water. This water softening process, however is costly and therefore undesired.
German Patent Publication DE-PS No. 855,452 discloses a method for the removal of ammonia from coking oven gas, in which a non-softened fresh water is used as a washing medium at the last washing stage. This known method provides that washing water containing the residuals of ammonia and obtained from the last stage of the washing process, is processed through a reaction space (settling tank) for a longer period of time to permit the separation of alkaline-earth carbonate and thereby at least a partial softening of water. This partially softened water must be then also supplied to the systems for another use, for example to a cold water circuit. This known process has not been used in practice for various reasons. On the one hand, the softening of washing water obtained in the known method is incomplete and takes a relatively long period of time whereby the utilization of such softened ammonia-containing water is limited. On the other hand, this known method appears to be necessary to decrease ammonia output because ammonia washed out from gas at the last stage of the washing process is in this case lost with washing water.